1. Field of the Invention
The present invention relates to a bidirectional optical module and more specifically, it relates to a bidirectional optical module to be included in an OTDR used in applications such as measurement of a fracture in an optical fiber communication network.
2. Description of the Related Art
A measuring device such as an optical fiber sensor, which executes measurement by using light in an optical communication system or the like, includes a light source that emits light and a light receiving unit that receives the light. A measuring device utilized in maintenance, management and the like of an optical communication system includes a light source that emits measurement light to be used for purposes of measurement to a measurement target optical fiber and a light receiving unit that receives light transmitted through the measurement target optical fiber.
For instance, an OTDR (Optical Time Domain Reflectometer) may be utilized in the installation, maintenance and the like of an optical fiber in order to monitor the state of the optical fiber through which light signals are transmitted for data communication in an optical communication system. An OTDR executes measurement to determine the conditions of the measurement target optical fiber, e.g., whether or not a disconnection has occurred at the measurement target optical fiber, the extent of loss or the like, by repeatedly inputting pulse light to the measurement target optical fiber and measuring the level of light reflected from the measurement target optical fiber, the level of light scattered to the rear and the length of time over which the light is received.
An OTDR includes a bidirectional optical module, a BIDI (bidirectional) module or the like, having a transmission unit and a reception unit housed in a single case. The advent of the FTTH (fiber to the home) technologies in recent years has resulted in these modules being offered at more affordable prices and thus, they have come to be used in a wide range of applications including other types of measuring devices and optical communication systems as well as OTDRs.
Patent reference literature 1 and patent reference literature 2 each disclose a structure that may be adopted in an OTDR equipped with a bidirectional optical module. The OTDR shown in FIG. 7, for instance, includes a bidirectional optical module 10, an LD drive unit 20, a sampling unit 30, a signal processing unit 40 and a display unit 50.
The bidirectional optical module 10 outputs pulse light to a measurement target optical fiber 73 via a measurement connector 60 and receives the light returning from the measurement target optical fiber 73. The LD drive unit 20 drives a light source disposed within the bidirectional optical module 10. The sampling unit 30 is a functional unit that converts an electrical signal (photocurrent) from a light receiving unit within the bidirectional optical module 10 to a voltage and samples the voltage resulting from the conversion. The signal processing unit 40 is a functional unit that engages the bidirectional optical module 10 to output pulse light via the LD drive unit 20 and engages the sampling unit 30 in a sampling operation. In addition, the signal processing unit 40 executes arithmetic operation processing on the electrical signal sampled by the sampling unit 30. The display unit 50 is a functional unit that indicates the signal processing results and may be constituted with, for instance, a display device.
As shown in FIG. 8, for instance, the bidirectional optical module 10 in the related art includes light emitting elements 11 and 13 that emit light, lenses 12 and 14 from which light is output as parallel light, a wave integrating element 15 that integrates a plurality of light fluxes into a single light flux, lenses 17 and 18 that condense light, a light branching element 16 that branches light and a light receiving element 19 that receives light (see patent reference literatures 1 through 3).
The light emitting elements 11 and 13 in the bidirectional optical module 10 may respectively emit light with a wavelength λ1 and light with a wavelength λ2. The light fluxes with the wavelengths λ1 and λ2 emitted from the light emitting elements 11 and 13 become parallel light fluxes through the lenses 12 and 14 respectively and these parallel light fluxes are then integrated at the wave integrating element 15. The light having become integrated at the wave integrating element 15 is then condensed via the lens 17, enters an optical fiber 71 and subsequently enters the measurement target optical fiber 73 connected to the optical fiber 71 via the measurement connector 60.
The light having entered the measurement target optical fiber 73 is reflected at a fracture or a connecting point in the measurement target optical fiber 73 and the returning light travels back through the measurement target optical fiber 73 to enter the bidirectional optical module 10 via the optical fiber 71 connected to the measurement target optical fiber 73 through the measurement connector 60. The returning light becomes parallel light at the lens 17 and then part of or all of the returning light is guided to the light receiving element 19 via the light branching element 16. The light reflected toward the light receiving element 19 via the light branching element 16 is condensed through the lens 18 before entering the light receiving element 19.
As an alternative, optical fiber couplers may be used to constitute a bidirectional optical module as shown in FIG. 9 (see, for instance, patent reference literature 4). An optical fiber coupler-type bidirectional optical module 10′ includes light emitting elements 11′ and 13′, a wave integrating optical coupler 15′, a light branching coupler 16′ and a light receiving element 19′. Light fluxes emitted from the two light emitting elements 11′ and 13′ enter the wave integrating optical coupler 15′ where they become integrated with each other. The integrated light then enters the measurement target optical fiber 73 connected through the measurement connector 60 via the light branching coupler 16′.
The light having entered the measurement target optical fiber 73 is reflected at a fracture or a connecting point inside the measurement target optical fiber 73 and the returning light travels back through the measurement target optical fiber 73 to enter the bidirectional optical module 10. The returning light is branched at the light branching coupler 16′ before it enters the light receiving element 19′. It is to be noted that one of the output ends, i.e. an output end a, the wave integrating optical coupler 15′ and one of the output ends, i.e., an output end b, of the light branching coupler 16′ are both treated so that no light is reflected at these ends.
Patent reference literature 1: Japanese Laid Open Patent Publication No. 2001-305017
Patent reference literature 2: Japanese Laid Open Patent Publication No. H4-296812
Patent reference literature 3: Japanese Laid Open Patent Publication No. H8-166526
Patent reference literature 4: Japanese Laid Open Patent Publication No. H10-336106
However, a phenomenon whereby stray light having been reflected or scattered from the surfaces of optical parts or other components such as metal cases, is repeatedly attenuated, tends to occur readily in the bidirectional optical module 10 in the related art adopting the structure shown in FIG. 10. The explanation is given here by assuming that a light receiving surface 19c of the light receiving element 19 includes a first light receiving area 19a located at the center of the light receiving surface 19c and a second light receiving area 19b ranging around the first light receiving area 19a and that better frequency characteristics are achieved over the first light receiving area 19a than over the second light receiving area 19b. While returning light 10a enters the first light receiving area 19a of the light receiving element 19, most of the stray light 10b enters the lens 18 at an angle different from that of the light 10a, as shown in FIG. 10. In this situation, the stray light 10b is condensed onto the second light receiving area 19b via the lens 18.
By measuring a fracture or a connecting point within the measurement target optical fiber 73 with the OTDR 1 equipped with the bidirectional optical module 10, a waveform such as that shown in FIG. 11 is detected. When a fracture or a connecting point is detected via the OTDR 1, the returning light registers a higher signal level, which manifests as a markedly protruding point such as point A in FIG. 11 in the waveform (a Fresnel reflection waveform). The stray light 10b received at the light receiving element 19 under such circumstances is bound to increase the extent of error in detecting the returning light level or the reflecting point position.
In particular, if a great deal of stray light 10b is received in the second light receiving area 19b, a problem arises in that the skirt range (skirt shaped range), also referred to as an attenuation dead zone, in the waveform representing the results of the measurement of light reflected at a fracture or a connecting point in the measurement target optical fiber 73 becomes greater. There are two types of stray light 10b, i.e., some of the light emitted from the light emitting elements 11 and 13 eventually ends up as stray light and some of the light returning from the measurement target optical fiber 73 also eventually ends up as stray light. The first type of stray light, which is received at the light receiving element 19 sooner than the returning light from the measurement target optical fiber 73, poses difficulty mainly when a reflecting point closer to the device needs to be detected. The second type of stray light is problematic in that with the stray light component added to the true returning light level, an error occurs in the detection of the returning light level and that the increased skirt range makes it more difficult to distinguish two reflecting points close to each other, which, in turn, makes it more difficult to identify loss.
An optical fiber in the optical fiber coupler-type bidirectional optical module 10′ constitutes a very narrow wave guiding passage and thus, returning light fluxes, intentionally coupled through a lens, are guided as integrated light. However, the optical fiber acts as a pinhole for stray light scattered at an angle different from the angle of the returning light, restricting the advance of the stray light. In other words, the structure of the optical fiber coupler-type bidirectional optical module 10′ does not allow redundant reflected light in the optical system or scattered stray light to enter the light receiving element 19′ readily. For this reason, while the skirt range in the waveform indicating the results of the measurement of light reflected at a fracture inside the measurement target optical fiber 73 executed by utilizing the optical fiber coupler-type bidirectional optical module 10′ remains small, the cost of the components of the optical fiber coupler-type bidirectional optical module 10′ is bound to be significant. In addition, since the form of the fiber is determined based upon the minimum flexural radius of the optical fiber, the module cannot be provided as a compact unit.